Femtochemistry on a nanoscale

By combining a femtosecond-laser with a low-temperature STM, we investigate femtochemistry down to single molecules. After imaging the adsorbed molecules and their adsorption geometry, the tip is retracted from the surface as far as possible to enable an ultra-short laser pulse to reach the sample without a change in the near field close to the surface. The pulse generates ’hot’ electrons in the metal. The increased eleectron density enables an energy gain for the adsorbed molecule to overcome reaction barriers at the surface, leading to e.g. dissociation or diffusion. The processes are determined and verified by imaging the same area at the sample surface as before the illumination. The measured dependence of the reaction rate on fluence gives a fascinating view of the mechanisms and the dynamics of the single steps of this kind of surface reactions. Our setup enables us to investigate these processes on single molecules and in dependence of surface defects or the presence of other molecules.

The experimental setup is described in Mehlhorn et. al, Rev. Sci. Instr. 78 (2007) 033908.